My dad used to say “useless as a rudder on a duck’s ass.”
Ouch. Just…ouch.
A glass hammer.
Mine too.
Not entirely useless, as it went to Michael Jackson.
…tits on a bull
“…a pile of Mymans.”
Useless as teats on a bull.
I’ve heard “boar pig”, which I thought the same of.
screen door on a submarine.
I think I’ve heard “…as a freezer to an Eskimo”
GAWD, there are a lot of responses involving the use of “tits.” (Which, in and of themselves, are not useless!)
I’d never heard this one until recently “The Naked Scientist” podcast put it to the test. They found a chocolate teapot would actually work for one batch of tea at least before it melted away. It made the tea pretty chocolatey though.
…a one-legged man at an ass-kicking.
I’ve only heard this one in the "“busy as…” constuction.
. . . a limp dick at the Playboy mansion.
I got to hear that one at a meeting and I’ve always wanted to repeat it but it’s hard to find a place it’s appropriate.
as useless as a rubber crutch
borrowing from another thread: “useless as a 4-way Yield-sign intersection”
Yes, I’ve heard it that way too. But it works as a simile in either case, wouldn’t you say? 
It doesn’t follow the “Useless as…” formulation, but one of my favorite phrases for describing someone (or something) as useless is “Not worth the powder to blow him up.”
…a bicycle for a fish.